Natural gas is a known alternative to combustion fuels such as gasoline and diesel. Much effort has gone into the development of natural gas as an alternative combustion fuel in order to combat various drawbacks of gasoline and diesel, including production costs and the subsequent emissions created by the use thereof. As is known in the art, natural gas is a cleaner burning fuel than other combustion fuels. Additionally, natural gas is considered to be safer than gasoline or diesel, as natural gas will rise in the atmosphere and dissipate, rather than settling.
To be used as an alternative combustion fuel, natural gas is conventionally converted into compressed natural gas (CNG) or liquified (or liquid) natural gas (LNG) for purposes of storing and transporting the fuel prior to its use. Conventionally, two of the known basic cycles for the liquefaction of natural gases are referred to as the “cascade cycle” and the “expansion cycle.”
Briefly, the cascade cycle consists of a series of heat exchanges with the feed gas, each exchange being at successively lower temperatures until the desired liquefaction is accomplished. The levels of refrigeration are obtained with different refrigerants or with the same refrigerant at different evaporating pressures. The cascade cycle is considered to be very efficient at producing LNG, as operating costs are relatively low. However, the efficiency in operation is often seen to be offset by the relatively high investment costs associated with the expensive heat exchange and the compression equipment associated with the refrigerant system. Additionally, a liquefaction plant incorporating such a system may be impractical where physical space is limited, as the physical components used in cascading systems are relatively large.
In an expansion cycle, gas is conventionally compressed to a selected pressure, cooled and then allowed to expand through an expansion turbine, thereby producing work as well as reducing the temperature of the feed gas. The low temperature feed gas is then heat exchanged to effect liquefaction of the feed gas. Conventionally, such a cycle has been seen as being impracticable in the liquefaction of natural gas since there is no provision for handling some of the components present in natural gas that freeze at the temperatures encountered in the heat exchangers, for example, water and carbon dioxide.
Additionally, to make the operation of conventional systems cost effective, such systems are conventionally built on a large scale to handle large volumes of natural gas. As a result, fewer facilities are built making it more difficult to provide the raw gas to the liquefaction plant or facility as well as making distribution of the liquefied product an issue. Another major problem with large-scale facilities is the capital and operating expenses associated therewith. For example, a conventional large-scale liquefaction plant, i.e., producing on the order of 70,000 gallons of LNG per day, may cost $16.3 million to $24.5 million, or more, in capital expenses.
An additional problem with large facilities is the cost associated with storing large amounts of fuel in anticipation of future use and/or transportation. Not only is there a cost associated with building large storage facilities, but there is also an efficiency issue related therewith as stored LNG will tend to warm and vaporize over time creating a loss of the LNG from storage. Further, safety may become an issue when larger amounts of LNG fuel product are stored.
In view of the shortcomings in the art, it would be advantageous to provide a process, and a plant for carrying out such a process, of efficiently producing liquefied natural gas on a relatively small scale. More particularly, it would be advantageous to provide a system for producing liquefied natural gas from a source after the removal of components thereof.
It would be additionally advantageous to provide a plant for the liquefaction of natural gas that is relatively inexpensive to build and operate, and that desirably requires little or no operator oversight.
It would be additionally advantageous to provide such a plant that is easily transportable and that may be located and operated at existing sources of natural gas that are within or near populated communities, thus providing easy access for consumers of LNG fuel.
Because there has been significant interest in liquefying natural gas recently, most technologies have focused on small-scale liquefaction where only a small portion of the incoming gas is liquefied with the majority of the incoming gas being returned to the infrastructure and source of the gas. These technologies work well in areas with established pipeline infrastructure for the return of gas from the small-scale liquefaction unit. Such small-scale units can be very cost effective, with liquefaction efficiencies significantly surpassing any full-scale production plant. Since the small-scale liquefaction units have a small footprint using little space, they are desirable for use with distributed gas supply systems. Also, small-scale liquefaction units typically have initial low capitol cost and low maintenance costs making it easier for such units to be purchased and operated.
Some locations do not have the benefit of a pipeline infrastructure, but still produce natural gas. Examples of types of such locations are waste disposal sites and coal bed methane wells, which typically produce enough natural gas to consider capturing and selling the gas in a convenient form. When the operators of waste disposal sites capture gas from the site, they can either use the gas for fuel of their equipment, or sell the fuel for other uses, thereby reducing costs of the waste disposal site. Coal bed methane wells can be productive over lengthy periods and the gas sold or used in onsite equipment.
However, without the ability to return natural gas to its source or an equivalent thereof, such as natural gas piping infrastructure, a conventional small-scale liquefaction unit is not feasible to use for natural gas liquefaction. Therefore, a compact natural gas liquefaction process and unit is needed that will provide complete liquefaction of the natural gas entering the process and unit. That is, 100% of the natural gas entering the process and unit or substantially all of the natural gas entering the process and unit may exit the unit as liquefied natural gas. If a small-scale complete liquefaction natural gas process and unit cannot be provided, it may not be feasible to liquefy natural gas from waste disposal sites and coal bed methane wells because conventional small-scale liquefaction processes and units require the return of un-liquefied natural gas from the unit to a pipeline infrastructure or other suitable receiving reservoir.
Complete liquefaction has long been the domain of large, capital intensive LNG plants, making it difficult for small natural gas markets to be conveniently supplied with natural gas. The use of complete liquefaction processes and apparatus as described herein facilitates liquefaction of natural gas at waste disposal sites, coal bed methane wells, and other types of single source supplies of natural gas where gas cannot be returned from the liquefaction process and apparatus. Other such instances where the use of the complete liquefaction process and unit described herein includes the liquefaction of natural gas from a pipeline where it is not desirable to return a large volume of natural gas from the liquefaction process and unit back into a pipeline because either the volume of natural gas to be returned to the pipeline is too great, or the pressure of the natural gas being returned to the pipeline is too great, or regulations prevent the return of natural gas from the conventional liquefaction process and unit to the pipeline, or policies prohibit the return of natural gas from the conventional liquefaction process and unit to a pipeline. The complete liquefaction processes and apparatus described herein facilitate the production of natural gas and the transportation thereof at locations previously considered to be unattractive for the production of natural gas.